1. Field of the Invention
This invention relates to the field of rockets and rocket thrusters, especially guided missiles equipped with divert and attitude rocket thrusters for controlling the flight path of the missiles.
2. Description of Related Art
Tactical aircraft, sea-skimming missiles, intercontinental ballistic missiles, theater missiles, and other tactical weapons are constantly being improved upon in their performance capabilities. As these tactical weapons become increasingly faster and evasive, highly maneuverable intercept missiles that are light in weight and have adequate range capability are required for targeting and successfully intercepting these tactical weapons.
The use of multiple stage rocket motors has long been known as an effective means for increasing propulsion efficiency in a missile system. In a multi-stage missile system, an upper stage carrying a payload sits atop one or more lower stage rocket motors. Usually, each rocket motor stage contains a case and a propellant or gas generant loaded in the case. The missile has a primary propulsion system that is typically initiated by igniting the lowest stage rocket motor. As the propellant or gas generant burns, the combustion products are rapidly expelled out the aft end of the motor to provide thrust to the missile. Upon burnout of the primary motor or at some other desired time, stage separation and thrust reduction of the separated stage may be effected. Stage separation includes detaching the lowest stage rocket motor from the remainder of the missile. Following detachment, the lowest rocket motor stage is physically isolated from contact with the upper stage. Separating the unneeded stage from the missile typically improves propulsion efficiency and maneuverability by reducing the mass of that subsequent rocket motor stages must propel. If the next higher stage contains a propellant or gas generant, that stage then becomes the operating lower rocket motor stage. The propellant of this new rocket motor lower stage may then be ignited to provide further propulsion to the missile. In this way, successive stages of rocket motors propel the payload toward its destination, with each stage being jettisoned as it is depleted of propellant or gas generant.
The guidance and control systems of missiles, especially those systems of the uppermost stage of a multi-stage rocket motor, possess enhanced missile guidance and control systems for improving their maneuverability and intercepting targets. The missile guidance and control system often contains radar or optics for detecting and pinpointing the location of a target. The missile guidance and control system takes this information and actuates thrust control systems for maneuvering to and successfully intercepting a target.
Because the lower and middle stages are often separated from the upper stage early in flight, the need for greater control and maneuverability over ballistic missiles is particularly prevalent at the upper stage of a ballistic missile. Oftentimes the trajectory of a ballistic missile must be corrected to insure accuracy in delivery of the payload or, in the case of multi-stage rocket weapons, precision final intercept maneuvering to counteract evasive movements of the target.
A highly maneuverable missile will typically include a thrust vector control (TVC) system, an axial thrust modulation control (TMC) system, and a roll control (RC) system, all interconnected and related.
The thrust vector control (TVC) systems rely on thrusters arranged radially or substantially radially relative to the main axis of the rocket motor for effecting sideways or “divert” movement to the missile. Thrust vector control systems have been developed using a variety of means, including movable or gimbaled nozzles, jet tabs, jet vanes and fluid injection into the nozzle. Typically, divert thrusters are provided in pairs of two or more, more commonly four or more, with each pair of thrusters diametrically opposed on opposite sides of the rocket motor case. Divert motors are perhaps most commonly arranged in a cruciform about the rocket motor axis, often at the axial center of gravity of the rocket motor. Actuation of divert thrusters allows missiles travelling at high speeds to be displaced sideways for enhancing intercept capabilities.
Usually, axial thrust modulation control is primarily attributed to a rear main thruster or a plurality of rear main thrusters arranged in a concentric pattern relative to the rocket motor axis. Axial thrust modulation control effectively manages and varies main axial nozzle thrust, thus controlling the velocity of the rocket.
Roll control systems typically comprise a pair or pairs of radial nozzles tangential to the rocket motor case. Selective actuation of the roll control thrusters may enhance stabilization of the rocket motor and facilitate imaging for guidance systems installed on the middle and upper stages of a rocket motor.
A single gas generant or propellant may feed all of the radial, tangential, and main thrusters of a rocket motor stage. Alternatively, for a given rocket motor stage, a main gas generant/propellant may be used for providing combustion products to the main thrust nozzles, and the radial and tangential thrusters may have a separate gas generant/propellant. In either case, pipes and valves couple the gas generator or solid propellant to the rocket thrusters. Independent and selective movement of the valves between open and closed positions controls the actuation and deactuation of the different thrusters or thruster pairs. By selectively controlling the amount of thrust or eliminating the thrust produced by the valves, axial thrust, divert movement, and roll are controlled during the flight for achieving precision maneuverability and enhancing the capability of the missile to intercept evasive targets.
Poppet valves have been used for selectively actuating and de-actuating rocket thrusters, and in particular divert thrusters. As referred to herein and understood in the art, a poppet valve is an on-off valve capable of movement to either a fully closed position or a single open position. An example of a poppet valve can be found in U.S. Pat. No. 3,330,114. A drawback to poppet valves, such as the one found in the aforementioned patent, is their incapability of being actuated to and retained at positions between the fully open and fully closed positions. That is, the poppet valve does not permit for proportional and continuously variable control over the effective throat area of a nozzle. Thus, controlling the amount of thrust generated by a poppet-valve thruster requires precision timing over the opening and closing movements of the poppet valve.
Another common valve found in thrusters is the pintle valve. Generally, a pintle valve is hydraulically, pneumatically, or electromechanically moved axially along the nozzle axis relative to the nozzle throat. As the pintle valve approaches the throat, the size of the throat passage is decreased, whereas movement of the pintle valve away from the throat increases the throat passage area. In this manner, thrust levels may be varied and controlled by axial movement of the pintle to a plurality of different axial positions. With some designs, the pintle valve provides flexibility by allowing for the possibility of continuously variable throat sizes.
However, the conventional pintle design has drawbacks. For example, the face of a pintle valve is subjected to a high load imparted by combustion products passing over the pintle valve and through the nozzle throat. In order to permit movement of pintle valves subjected to such loads, the pintle actuation mechanisms must have sufficient horsepower to overcome the load imparted to the pintle valve by the combustion products. In order to generate sufficient horsepower, the actuation mechanism often must be of a rather large size and heavy mass. However, the size and weight penalties contributed by large actuation mechanisms adversely affect the performance and maneuverability of the rocket motor.